The present invention relates to powder metallurgy (P/M) stainless steel powders and compacts therefrom, and more particularly to maximizing the corrosion resistance of tin containing stainless steel powder compacts.
Heretofore, poor corrosion resistance of stainless steel powder compacts has been attributed mainly to the porosity within the compacts, thus most techniques for overcoming corrosion problems have been aimed at closing the porosity. Prior techniques aimed at minimizing the surface porosity of the compacts made from such stainless steel powders include mechanical closure treatment, plastic impregnation, surface coatings, or passivation techniques. Each of these techniques has some limitation as to its effectiveness, in addition to raising the cost of the final product. Other proposals have aimed at improving the corrosion resistance of stainless steel powder compacts by changing the compacting and sintering parameters. These proposals generally state that the sintering conditions and sintering atmosphere have a marked influence on the corrosion properties of the powder compact; however, most of the experimental results reported in these proposals are inconsistent. For example, Kalish and Mazza ("An Evaluation of Dissociated Ammonia and Hydrogen Atmospheres for Sintering Stainless Steel", Journal of Metals, Trans. AIME, February 1955, pages 304-310) state that sintering in hydrogen provides a more corrosion resistant compact than sintering in dissociated ammonia. Stosuy et al (Metal Progress, Vol. 91, pages 81-85, 1967) and Jones ("The Effect of Processing Variables on the Properties of Type 316-L Powder Compacts", Progress in Powder Metallurgy, Vol. 30, pages 25-50, April, 1974) report that an optimum combination of mechanical properties and corrosion resistance of the compact can be obtained by sintering the compact in dissociated ammonia. Furthermore, Sands et al ("The Corrosion Resistance of Sintered Austenitic Stainless Steel", Modern Developments in Powder Metallurgy, Vol. 2, H. H. Hausner, ed., Plenum Press, New York, N.Y., pages 73-83, 1966) report that while sintering in vacuo always gives a good corrosion resistant product, sintering in either dissociated ammonia or hydrogen can lead to loss of corrosion resistance.
Japanese Tokkai 52-35708, now Japanese Pat. No. 79-29285 to Daido Steel Company, Ltd., reports that the addition of a small amount of tin and optionally tin plus copper to stainless steel powder during atomization or prior to compacting while maintaining a regular rear spherical shape for the powder can improve the corrosion resistance of stainless steel powder compacts made therefrom. Certainly, the inconsistencies among these various citations demonstrate that confusion is prevalent in the art and prevent the making of any generalizations--or the combination of teachings with regard to the corrosion resistance of stainless steel powder and compacts made therefrom.
In accord with the present invention, it has now been discovered that stainless steel powders atomized in an oxidizing atmosphere (e.g. a conventional water atomization process) are very irregular and surface-enriched in silicon oxides (primarily silicon dioxide) and, thus, surface depleted in chromium. Further, if this stainless steel powder contains tin, the corrosion resistance of such alloys can be maximized if the powder compacts are sintered to produce a critical tin:silicon ratio on the surface of the powder compact. One advantage of the present invention is that it provides a method of improving corrosion resistance without adding processing steps. Another benefit is that expensive alloying additions such as molybdenum can be replaced by less expense alloying additions of tin or tin plus copper. Another benefit of the present invention is that the compressibility of stainless steel powders can be enhanced. Still other benefits will be obvious from the detailed description of the invention.